It is generally known that thermosetting compositions are prepared from compounds having carboxyl groups and compounds having reactive functional groups which can form chemical bonds with the carboxyl groups by heating, such as epoxy group, oxazoline group, silanol group, alkoxy silane group, amino group, imino group, isocyanate group, blocked isocyanate group, cyclocarobonate group, vinyl ether group, vinyl thioether group, aminomethylol group, alkylated aminomethylol group, acetal group and ketal group.
As the thermosetting compositions described above, for example, compositions comprising combination of carboxyl groups and epoxy groups have been disclosed in Laid Open Japanese Patent Application Showa 51-114429, Laid Open European Patent Application 29595 and U.S. Pat. Nos. 4,371,667, 4,650,718, 4,681,811, 4,703,101 and 4,764,430.
The compositions comprising combination of carboxyl groups and oxazoline groups have been disclosed in U.S. Pat. No. 3,505,297 and Laid Open Japanese Patent Applications Showa 60-88038 and Heisei 2-115238.
The thermosetting compositions give cured products having excellent chemical properties, physical properties and weathering resistance and are widely utilized in the field of coatings, ink, adhesives and molded articles.
However, the reactivity between the carboxyl group and the reactive functional groups is high so that compositions in which compounds having carboxyl groups and compounds having the reactive functional groups exist together have problems that the compositions are gelled during storage and the period suitable for pot life is short.
The compounds comprising carboxyl groups conventionally utilized for thermosetting compositions described above have problems that solubility to generally used organic solvents is low and that compatibility to the compounds having reactive groups, which react with the carboxyl group, is inferior because of the property of the carboxyl group capable of forming strong hydrogen bonds. When the thermosetting compositions are utilized as top coating compositions, there are naturally have problems that preparation of coating composition of high solid content is difficult and that appearance of the finished coat is inferior.
For the purpose of solving these problems, it was proposed that carboxyl group was blocked by converting it to tertiary-butyl ester and the ester was decomposed by heating, and the free carboxyl group was regenerated by elimination of isobutene (Laid Open Japanese Patent Application Heisei 1-104646).
However, there were problems that this method required a high temperature, such as 170-200.degree. C., for the heat decomposition of the tertiary-butyl ester and that pock marks which were made by foaming and degassing of isobutene formed by the decomposition remained on the cured surface of the coating. Thus, this method is not sufficiently satisfactory.
The inventors have already suggested a thermosetting composition comprising a compound having functional groups in which carboxyl groups are blocked with vinyl ethers, and a compound having reactive functional groups which react the functional groups (Laid Open European Patent Application 643112) in order to solve the problems described above. But, a thermosetting composition which gives more excellent paint film having impact resistance, processability and elasticity is needed in some uses.
On the other hand, a polymer of a vinyl monomer such as a (meth) acrylic ester polymer and a styrene polymer, and a polyester resin are nowadays mainly utilized as a polymer of a main material in the thermosetting composition utilized in the field of coatings.
The polymer of the vinyl monomer is generally excellent in hardness, chemical resistance such as alkali resistance and acid resistance, stain resistance to carbon powder, transparency and gloss. Therefore, the polymer of the vinyl monomer is broadly used as polymers for main materials of coating compositions. But, the polymer has defects of brittleness, poor processability and weak impact strength though the polymer has high hardness.
The polyester resin is widely used as polymers for main materials of coating compositions because the polyester resin generally has elasticity and is excellent in processability and transparency. But, the polyester resin has defects that chemical resistance such as acid resistance and stain resistance to various stain substances such as carbon powder, oil ink, rain and foods are inferior.
As the means for overcoming the both defects and making use of both characteristics, it is thought to blend the polymer of vinyl monomer and the polyester resin. But, they have defects that phase separation is caused and that required hardness, chemical resistance and processability are not obtained and simultaneously transparency of the composition is impaired, because the polymer of vinyl monomer and the polyester resin having high molecular weight are poor in compatibility each other.
The present invention has an object to solve the defects relating to storage stability, appearance during coating and preparation of high solid content coating composition, which exist in the conventional thermosetting composition comprising the conventional carboxyl group-containing compound and the compound having reactive functional group which can form chemical bonds with the carboxyl group. The present invention also has an object to provide thermosetting compositions which can give cured products having excellent hardness, chemical resistance and stain resistance originated from the polymer of vinyl monomer, and processability and impact resistance originated from the polyester resin, and further excellent elasticity and can be preferably utilized in the field of, for example, coatings, ink, adhesives and molded articles. Other objects of the invention are to provide methods of coating which use the thermosetting compositions as top coating compositions and can give a paint film having film properties described above and excellent appearance to the finished articles, and to provide coated articles.
Extensive investigations were undertaken by the present inventors in order to develop the thermosetting compositions having the preferable properties described above lead to discoveries that it is preferable to utilize (A) an epoxy group-containing modified polyester polymer produced by a specific process for preparation as the main material and the objects can be achieved by a composition comprising, essentially the polymer (A) and (B) a compound having in the molecule two or more carboxyl groups which are blocked by a specific vinyl ether group, vinyl thioether group or heterocyclic group having vinyl type double bond and oxygen or sulfur as the hetero atom component, and optionally (C) a thermal latent acid catalyst and/or (D) a dispersing component of at least one inorganic oxide sol selected from the group consisting of aluminum oxide sol, silica sol, zirconium oxide sol and antimony oxide sol. The present invention was completed on the basis of the discoveries described above.